Familiar arch shape cross section molded plastic corrugated leaching chambers are buried in soil for use. They have opposing sidewalls which run upwardly and inwardly from opposing side base flanges to a solid top. Chambers must be strong enough to resist the overlying load of soil and any objects such as vehicles which move across the soil surface. The chamber sidewalls have commonly been perforated for flow of water laterally into the surrounding soil. The chamber construction or installation should hinder soil particulates from moving into the chamber interior and allow good outward flow of wastewater. In some situations that aim is achieved by the configuration of the chamber itself. For example, the chamber has downward sloping slots. In other situations, the chamber has holes which are not suited to inhibiting soil movement, and a barrier sheet such as geotextile filter fabric is interposed between the soil and chamber, as discussed further below. Generally, it is desirable to maximize sidewall leaching area per unit length of chamber. The leaching area of a chamber comprises both the exposed soil area at the base of the chamber and the water-exposed soil area adjacent the sidewalls of the chamber.
In one typical type of injection molded chamber, the peak and valley corrugations have downward sloping slots which are defined by louvers. See U.S. Pat. No. 7,189,027 of Brochu et al., where the chamber has a continuous curve arch. See U.S. Pat. No. 4,759,661 of Nichols et al. where the chamber has planar sidewalls and a generally trapezoid shape cross section. Louvers which define slots have often been structural parts of the sidewalls; first, in being a part of the sidewall structure which transfers the down-load of overlying soil to the base flanges; and, second, in resisting the lateral or inward-force of soil. There are countervailing aims when there are slotted sidewalls. The essential aim is to maximize the exposed soil area into which wastewater may flow. On the one hand, the openings should be large, and the down-slope small, so there is a resultant large open area, to maximize the outflow of water. On the other hand, the openings should be small and the down-slope steep, to minimize inflow of particles, and maximize the structural plastic per unit area in the sidewall. A practical design compromise is typically reached. See U.S. Pat. Nos. 5,511,903, 7,189,027 and other patents referenced herein for examples.
Other kinds of molded plastic leaching chambers have spaced apart round or of other shape openings. For example, chambers made by thermoforming plastic sheet have such kind of sidewall perforations, which typically might be made in a secondary operation. See U.S. Pat. No. 5,087,151 of Ditullio for a thermoformed chamber with round sidewall holes. Round holes of substantial size generally necessitate the use of geotextile draped over the chamber, before a leaching trench is backfilled with common soil. Holes must not be so large that that the force of soil can push geotextile through the opening. Typically, non-slotted holes are of the order of 0.5-1 inch diameter. In some installations, a geotextile filter fabric is laid over a slotted sidewall chamber, to inhibit inflow of fine sand particles. For example, see U.S. Pat. No. 7,207,747 of England, which describes how geotextile is either wrapped around a chamber or run lengthwise along the chamber sidewalls.
There is a continuing aim to lower the cost of leaching chambers, such as by decreasing the weight of plastic, thus reducing manufacture cost, while maintaining strength and adequate sidewall leaching area; or by decreasing the cost of molds and molding. Different soils may indicate different sidewall opening configurations. If openings in a chamber sidewall are made small to resist fine particulates, leaching area per unit length is reduced and such chamber will be inefficient in coarse soils, e.g., more small-opening chambers are needed to meet a particular total leaching area than would be the case if the sidewall was tailored to a coarse material.
While different chamber models might be molded, each having a particular sidewall configuration, that would mean not only increased mold costs and inventory costs, but this approach is impractical in the market place. When, as described above, geotextile is placed over the whole of a leaching chamber, labor and material costs are increased. Thus, there is a need to address these issues in an economic way.